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                <p>[TOC]</p>
<h3 id="一、java中异常分类及异常捕获"><a href="#一、java中异常分类及异常捕获" class="headerlink" title="一、java中异常分类及异常捕获"></a>一、java中异常分类及异常捕获</h3><p>众所周知，异常处理的方式有两种：第一、手动抛出异常；第二，捕获异常；抛出异常可以分为显式和隐式两种，显式抛异常是指在代码中使用“throws”关键字，手动将异常抛出；隐式异常的控制主体是java虚拟机，它指虚拟机在执行过程中，碰到无法继续执行的异常，而自动抛出异常，例如，java虚拟机在读取素组时，发现输入的索引是负数，因此java虚拟机会抛出素组角标越界异常（ArrayIndexOutOfBoundsException）。在代码中捕获异常涉及如下三种代码块：</p>
<ol>
<li>try{}代码块：用来标记要进行异常监控的代码，也就是告诉虚拟机这个块范围内的代码需要异常监控。</li>
<li>catch（）{}代码快：跟在try{}代码块之后，用来捕获在try代码块中触发的某种catch()指定类型的异常。除了声明所捕获的异常类型之外，catch代码块还定义了针对捕获的异常类型发生后，该如何处理的区域，总的来说就是try代码块中如果发生了异常，被虚拟机监控到了，并且异常类型与catch()中定义的异常类型匹配，那么就可以在catch(){}的大括号中进行处理。在java代码中，try代码块后面可以跟多个catch代码块，用来捕获不同类型的异常，java虚拟机会从上到小匹配异常处理器，因此，定义在前面的catch代码块所捕获的异常类型不能覆盖后面的，否则编译器会报错(因为你有多个触发器，虚拟机都不知道交给谁去处理了)。</li>
<li>finally{}代码块：跟在try代码块和catch代码块之后，用来声明一段必须运行的代码。它的设计初衷是为了避免跳过某些关键的清理代码，例如关闭已经打开的系统资源。</li>
</ol>
<p>在程序正常运行，没有抛出异常的情况下，finally代码块会在try包裹的代码执行完毕后执行；又或者，在执行try代码块中的代码发生了异常，如果异常没有捕获(也就是没有哦catch代码块)，finally代码块会先被运行，并在运行结束后抛出异常；又或者包含catch代码块，用于捕获异常，finally代码块会在catch代码块包裹的内容执行完毕后运行；在某些极端情况下，执行catch代码块内容过程中触发了异常，那么finally代码块同样会运行，运行完毕后会抛出catch中发生的异常；又或者，在某些更极端的情况下，finally代码块执行过程中发生了异常，那么只好中断finally块执行，并抛出异常；只有在一种情况下finally块不会被执行，那就是虚拟机进程终止，例如System.exit()；上面的这些情况听起来有点绕，但是等我讲完后面java虚拟机异常处理机制后，你便会明白其中的所以然。</p>
<h3 id="二、异常的基本概念"><a href="#二、异常的基本概念" class="headerlink" title="二、异常的基本概念"></a>二、异常的基本概念</h3><p>在java语言规范中，所有异常都是Throwable类或器子类的实例，Trowable有两大直接子类，第一个是Error，这部分异常是虚拟机内部运行过程中出现的错误，代码是无法捕获的，例如OOM，栈溢出等，当程序发生Error错误时，它的执行状态已经无法恢复，需要终止当前线程甚至终止虚拟机进程；第二是Exception，涵盖程序可以捕获，并处理的异常，大致的类图如下所示：</p>
<p><img src="https://i.loli.net/2020/10/18/RpGoM16U57KzJwf.png" alt="image-20201018151711858"></p>
<p>Exception下有一个特殊的子类RuntimeException,用来表示，“程序报错了，但是可以通过捕获异常自己处理”，RuntimeException和Error属于java中的非检查异常，也就是java编译器在编译器无法发现，它们是在程序运行过程中，根据数据和环境的变化，抛出来的异常；其它的在编译期能检测出来的异常则属于检查异常；在java语法中，所有的检查类异常都需要程序显示地进行捕获，或者在方法声明中使用throws关键字抛出给上层调用者处理。</p>
<p>在了解了异常的相关概念后，那么异常在虚拟机中是否会消耗性能呢？<code>异常实例的构造十分昂贵，这是由于在构造异常实例时，java虚拟机需要生成该异常的栈轨迹，该操作会逐一访问当前线程的java栈帧，并且记录下各种调试信息，包括栈帧所执行的方法名，方法所在的类名、文件名、以及在代码中第几行触发该异常的，其实在实际开发过程中也深有体会，控制台一般会先抛出异常发生点的信息，然后打印上层调用者的信息，然后再打印上上层调用者的信息，就像冒泡一样，所以在当前线程调用的方法栈中，要遍历当前线程栈中压栈了的整个调用链条上的方法，并记录相关信息，所以从表象上看肯定是需要消耗性能的</code>。既然异常实例的构造那么代价那么昂贵，我们是否可以缓存异常实例，在需要时直接抛出呢？从语法的角度上是可以的，但是这样就会带来问题，缓存的异常实例中的栈轨迹并未当前调用栈的轨迹，所以这种情况抛出的异常会误导开发人员，也就是为什么我们在实际的开发中不适用缓存，而使用创建新的异常实例的原因。</p>
<h3 id="三、java虚拟机是如何捕获异常的"><a href="#三、java虚拟机是如何捕获异常的" class="headerlink" title="三、java虚拟机是如何捕获异常的"></a>三、java虚拟机是如何捕获异常的</h3><p>在编译器编译生成的字节码文件中，每一个方法都附带一个异常表，异常表信息都随着编译完成写入到字节码文件中。异常表的每一行条目代表一个异常处理器，<code>每一行都由from指针、to指针、target指针、所捕获的异常类型构成</code>，这些指针的值用于定位字节码，例如from指针为1，to指针为10,异常处理代码块起始位置为11,当从指针为1位置到指针为10位置之间发生了异常，如果异常类型和所捕获的异常类型一致，那么跳转到指针为11的位置，进行运行。其中，from指针和to指针标识了异常处理器监控的范围，例如try代码块所覆盖的范围，target指针则指向异常处理器的起始位置,例如catch代码块的起始位置，下面我们来通过一个例子分析：</p>
<p><img src="https://i.loli.net/2020/10/18/d2WUgxJPpYnfb9w.png" alt="image-20201018154053777"></p>
<p>上面的代码，经过编译后，该方法的异常表(异常表是与方法关联的)拥有一个条目，其中from指针和to指针分别为0和3，代表异常的监控范围从索引0开始，到索引3的字节码结束(不包括3)，该条目的target指针为6，代表这个异常处理器从索引6的字节码开始，条目的最后一列，代表该异常处理器所捕获的异常类型为Exception;当异常触发时，java虚拟机会从上到下遍历异常表中的所有条目行，当触发异常的字节码的索引范围在与异常表的某一行匹配，java虚拟机会判断所抛出的异常和该条目要捕获的异常是否匹配，如果匹配，java虚拟机会将控制流程跳转到target指针指向的字节码位置，并执行。</p>
<p>如果遍历完方法所有的异常条目，java虚拟机仍未匹配到异常处理器，那么虚拟机会弹出当前方法对应的java栈帧，并在上层调用者中重复上诉操作，以此类推，最坏的情况就是，java虚拟机需要遍历当前线程调用栈中国所有方法的异常表，如果遍历到当前线程方法栈的最底下的方法，此时如果还没有匹配到异常处理器，那么异常会跑给java虚拟机，由java虚拟机抛出异常。</p>
<h3 id="四、站在字节码的角度分析finally代码块是如何执行的"><a href="#四、站在字节码的角度分析finally代码块是如何执行的" class="headerlink" title="四、站在字节码的角度分析finally代码块是如何执行的"></a>四、站在字节码的角度分析finally代码块是如何执行的</h3><p>finally代码块的编译比较复杂，当前java编译器的做法，是复<code>制finally代码块的内容</code>，分别放在try-catch代码块所有<code>正常执行路径以及异常执行路径的出口</code>，也就是为了保证finally一定执行，需要将try-catch代码块执行完毕后的出口用finally块堵住。</p>
<p><img src="https://i.loli.net/2020/10/18/qtkCaMVSUnzmYTu.png" alt="image-20201018155748111"></p>
<p>如果包含finally块，针对异常执行路径，java编译器会生成一行或多行异常表条目，用于监视整个try-catch代码块，并且捕获所有种类的异常（在字节码的异常表中以any类型匹配所有异常类型），这些异常表条目的target指针将指向另外一份复制的finally代码块(字节码中finally块会被复制多份)，并且，在finally块的最后，java编译器会重新抛出所捕获的异常。</p>
<p>下面通过javap工具来查看下面这段包含try-catch-finally代码块的字节码，为了更好区分每一段，我定义了四个实例字段：tryBlock、catchBlock、finallyBlock、methodExit，并且在对应的代码块中赋值这些字段。</p>
<pre class=" language-java"><code class="language-java"><span class="token comment" spellcheck="true">//类文件内容</span>
<span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Foo</span> <span class="token punctuation">{</span>
    <span class="token keyword">private</span> <span class="token keyword">int</span> tryBlock<span class="token punctuation">;</span>
    <span class="token keyword">private</span> <span class="token keyword">int</span> catchBlock<span class="token punctuation">;</span>
    <span class="token keyword">private</span> <span class="token keyword">int</span> finallyBlock<span class="token punctuation">;</span>
    <span class="token keyword">private</span> <span class="token keyword">int</span> methodExit<span class="token punctuation">;</span>

    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">test</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">try</span> <span class="token punctuation">{</span>
            tryBlock <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span><span class="token class-name">Exception</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
            catchBlock <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span> <span class="token keyword">finally</span> <span class="token punctuation">{</span>
            finallyBlock <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
        methodExit <span class="token operator">=</span> <span class="token number">3</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token comment" spellcheck="true">//反编译字节码文件</span>
C<span class="token operator">:</span>\Users\Administrator\Desktop<span class="token operator">></span> javac  Foo<span class="token punctuation">.</span>java
C<span class="token operator">:</span>\Users\Administrator\Desktop<span class="token operator">></span> javap <span class="token operator">-</span>c Foo
Compiled from <span class="token string">"Foo.java"</span>
<span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Foo</span> <span class="token punctuation">{</span>
  <span class="token keyword">public</span> <span class="token function">Foo</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    Code<span class="token operator">:</span>
       <span class="token number">0</span><span class="token operator">:</span> aload_0
       <span class="token number">1</span><span class="token operator">:</span> invokespecial #<span class="token number">1</span>                  <span class="token comment" spellcheck="true">// Method java/lang/Object."&lt;init>":()V</span>
       <span class="token number">4</span><span class="token operator">:</span> <span class="token keyword">return</span>

  <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">test</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    Code<span class="token operator">:</span>
       <span class="token number">0</span><span class="token operator">:</span> aload_0
       <span class="token number">1</span><span class="token operator">:</span> iconst_0
       <span class="token number">2</span><span class="token operator">:</span> putfield      #<span class="token number">2</span>                  <span class="token comment" spellcheck="true">// Field tryBlock:I</span>
       <span class="token number">5</span><span class="token operator">:</span> aload_0
       <span class="token number">6</span><span class="token operator">:</span> iconst_2
       <span class="token number">7</span><span class="token operator">:</span> putfield      #<span class="token number">3</span>                  <span class="token comment" spellcheck="true">// Field finallyBlock:I</span>
      <span class="token number">10</span><span class="token operator">:</span> <span class="token keyword">goto</span>          <span class="token number">35</span>
      <span class="token number">13</span><span class="token operator">:</span> astore_1
      <span class="token number">14</span><span class="token operator">:</span> aload_0
      <span class="token number">15</span><span class="token operator">:</span> iconst_1
      <span class="token number">16</span><span class="token operator">:</span> putfield      #<span class="token number">5</span>                  <span class="token comment" spellcheck="true">// Field catchBlock:I</span>
      <span class="token number">19</span><span class="token operator">:</span> aload_0
      <span class="token number">20</span><span class="token operator">:</span> iconst_2
      <span class="token number">21</span><span class="token operator">:</span> putfield      #<span class="token number">3</span>                  <span class="token comment" spellcheck="true">// Field finallyBlock:I</span>
      <span class="token number">24</span><span class="token operator">:</span> <span class="token keyword">goto</span>          <span class="token number">35</span>
      <span class="token number">27</span><span class="token operator">:</span> astore_2
      <span class="token number">28</span><span class="token operator">:</span> aload_0
      <span class="token number">29</span><span class="token operator">:</span> iconst_2
      <span class="token number">30</span><span class="token operator">:</span> putfield      #<span class="token number">3</span>                  <span class="token comment" spellcheck="true">// Field finallyBlock:I</span>
      <span class="token number">33</span><span class="token operator">:</span> aload_2
      <span class="token number">34</span><span class="token operator">:</span> athrow
      <span class="token number">35</span><span class="token operator">:</span> aload_0
      <span class="token number">36</span><span class="token operator">:</span> iconst_3
      <span class="token number">37</span><span class="token operator">:</span> putfield      #<span class="token number">6</span>                  <span class="token comment" spellcheck="true">// Field methodExit:I</span>
      <span class="token number">40</span><span class="token operator">:</span> <span class="token keyword">return</span>
    Exception table<span class="token operator">:</span>
       from    to  target type
           <span class="token number">0</span>     <span class="token number">5</span>    <span class="token number">13</span>   Class <span class="token class-name">java</span><span class="token operator">/</span>lang<span class="token operator">/</span>Exception
           <span class="token number">0</span>     <span class="token number">5</span>    <span class="token number">27</span>   any
          <span class="token number">13</span>    <span class="token number">19</span>    <span class="token number">27</span>   any
<span class="token punctuation">}</span>
</code></pre>
<p>可以看到，编译结果包含三分finally代码块，其中，前了两份分别位于try代码块和catch代码块的正常执行路径出口（如果try中没有发生异常，则会执行第一个finally块，如果try发生了异常，被catch捕获，在执行catch过程中没有发生异常，则最后回指定第二个finally块），最后一份则是作为异常处理器，监控try代码块以及catch代码块，它将捕获try代码块触发的，未被catch代码块捕获的异常，以及catch代码块触发的异常。</p>
<p>这里有一个问题，如果catch代码块捕获了异常，并且触发了另外一个异常，那么finally捕获并且抛出的异常是哪个呢？答案是后者，也就是说原本的异常会被忽略掉，这对代码调试来说十分不利。</p>
<h3 id="五、java7提供的Supressed异常及语法糖"><a href="#五、java7提供的Supressed异常及语法糖" class="headerlink" title="五、java7提供的Supressed异常及语法糖"></a>五、java7提供的Supressed异常及语法糖</h3><p>基于上面提到的如果在执行catch过程中出现了异常，最终finally块在执行完毕后抛出的是新的一个异常，catch()中的异常就被吃掉了，这对调试不利，因此java7中引入了Supressed异常来解决这种问题，这个特性允许开发人员键给一个异常附于另一个异常之上，因此上这种情况，使用Supressed异常，可以在finally执行完毕后会跑出catch()捕获的异常信息，及catch(){}块中发生的异常信息；然而在java语法层的finally代码块缺少指向所捕获的异常的引用，所以supressed异常结合finally来用非常繁琐，为了解决这种繁琐的操作，java7 专门引入了try-with-resources语法糖，通过编译时增强，在编译层的字节码上自动织入了supressed异常字节码。</p>
<p>为了说明语法糖的好处，接下来我引入一个资源关闭的例子，在传统的try-finally代码块中关闭资源的代码如下：</p>
<p><img src="https://i.loli.net/2020/10/18/UPcxeK1DvpNnT9S.png" alt="image-20201018164559470"></p>
<p>在java7中可以使用try-with-resources语法糖，优化如上代码：</p>
<p>1、程序在try关键字后声明并实例化实现了AutoCloseable接口的类，编译器将自动织入close()方法的操作,在声明多个AutoCloseable实例的情况下，编译生成的字节码类似于上面手动编写代码的编译结果，与手动编写的代码相比，try-with-resource语法糖还会使用supressed异常功能，来避免上面说到的异常被吃掉的情况。</p>
<p><img src="https://i.loli.net/2020/10/18/28r6kZDGe17mdAP.png" alt="image-20201018165220556"></p>
<p><img src="https://i.loli.net/2020/10/18/5MaSJiGhOBVoPNn.png" alt="image-20201018165234094"></p>
<p>除了try-catch-resources语法糖，java7还支持同一个catch中捕获多中异常，实现原理非常简单，在字节码层面的异常表中生成多行条目即可：</p>
<p><img src="https://i.loli.net/2020/10/18/MlxpkeXaVzZYu16.png" alt="image-20201018165428473"></p>
<h3 id="六、总结"><a href="#六、总结" class="headerlink" title="六、总结"></a>六、总结</h3><p>今天主要介绍了java虚拟机的异常处理机制，java异常分为Exception和Error两种，而Exception又分为RuntimeException和其它类型。RuntimeException和Error是非检查型异常，其它的Exception都是检查型异常，在触发时，需要显示捕获，或者在方法定义上使用throws关键字抛出。</p>
<p>java字节码中，每一个方法都有其对应的一个异常表，当程序触发异常时，java虚拟机将查找异常表，并依次决定需要将控制流转至哪一个异常处理器中，java代码中的catch代码块和finally代码块都会生成异常表条目。</p>
<p>java7中引入了supressed异常、try-with-resources语法糖、多异常捕获等新特性，后面的两个属于语法糖，能极大精简我们的代码。</p>

                
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